1) Field of the Invention
The present invention relates to a protective film for use in sealing or protection of various electronic devices such as organic electroluminescence devices, etc. used in light emitting devices, etc. in various display devices or light sources or backlight in display devices or optical communication systems, and a process for producing the same.
2) Related Art of the Invention
Generally, electronic devices are protected by sealing to some extent, because they are highly susceptible to external circumstance factors such as moisture, heat, etc., and recent trends toward higher density and higher precision of electronic devices are requiring highly reliable sealing, and thus intensive studies have been so far fostered.
Materials so far used for this purpose are shifting from the inorganic system to the organic/inorganic complex systems taking the versatility of organic materials into consideration. It is now an important problem how to suppress degradation of organic materials very easily susceptible to external circumstance factors such as moisture, heat, stress, etc.
It is organic electroluminescence devices that have recently attracted attention among devices using such organic materials.
Electroluminescence device is a light-emitting device based on electroluminescence of solid fluorescent substances, and inorganic electroluminescence devices based on inorganic light-emitting materials have been so far practically applied to the backlight of liquid crystal display, flat display, etc.
Electroluminescence devices based on organic materials, on the other hand, have been long studied in various ways, but very poor light-emitting efficiency has been a bottleneck to full scale practical application study.
However, C. W. Tang et al of Eastman Kodak Co. proposed in 1987 an organic electroluminescence device in a functionally separated type, stacked layer structure, where organic materials were divided into two layers, i.e. a hole transport layer and a light-emitting layer, and disclosed that a high luminance e.g. 1,000 cd/m2 or more could be obtained even at a low voltage such as 10 V or less [C. W. Tang and S. A. Vanslyke: Appl. Phys. Lett., 51 (1987), 913, etc.]. After the disclosure the organic electroluminescence devices were suddenly highlighted and extensive studies have been made of organic electroluminescence devices in a similar functionally separated type, stacked layer structure, some of which are now practically used.
Explanation will be made below of the conventional organic electroluminescence device, referring to FIG. 7.
FIG. 7 is a cross-sectional view showing the essential part of the conventional organic electroluminescence device, where reference numerals have the following meanings: 1: substrate, 2: anode, 3: organic thin film layer, 4: hole transport layer, 5: light-emitting layer and 6: cathode.
As shown in FIG. 7, the conventional organic electroluminescence device comprises transparent or opaque substrate 1 of glass, etc., anode 2 made from a transparent conductive film of ITO, etc. formed on substrate 1 by sputtering, resistance-heated vapor deposition, etc., hole transport layer 4 made from N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-1,1xe2x80x2-diphenyl-4,4,-diamine (hereinafter referred to as TPD), etc., formed on anode 2 by resistance-heated vapor deposition, etc., light-emitting layer 5 made from 8-hydroquinoline aluminum (hereinafter referred to as Alq3), etc. formed on hole transport layer 4 by resistance-heated vapor deposition, etc., and cathode 6 made from a metal film, etc. formed on light-emitting layer 5 by resistance-heated vapor deposition, etc., where hole transport layer 4 and light-emitting layer 5 constitute organic thin film layer 3 in FIG. 7.
When a DC voltage is applied or direct current is passed between anode 2 as a plus electrode and cathode 6 as a minus electrode in the organic electroluminescence device in the stacked layer structure, holes are injected into light-emitting layer 5 from anode 2 through hole transport layer 4 and electrons are injected into light-emitting layer 5 from cathode 6. In light-emitting layer 5, holes and electrons are recombined to form excitons and when the excitons thus formed are shifted from the excited state to the ground state light emission phenomena take place. Light emission wavelength can be changed by changing the stacked layer structure of organic thin film layer 3 or materials of light-emitting layer 5.
To improve light emission characteristics of such an organic electroluminescence device, studies have been made of 1) improvement of the structure of organic thin film layer, i.e. light-emitting layer, hole transport layer, etc. or organic materials for these layers and 2) improvement of anode and cathode materials.
For example, to lower the barrier between the cathode and the light-emitting layer, thereby facilitating injection of electrons into the light-emitting layer in case of 2), materials of small work function and high electroconductivity, e.g. Mgxe2x80x94Ag alloys (U.S. Pat. No. 4,885,211), Alxe2x80x94Li alloys (JP-A-5-121172), etc. were proposed, and these materials have been widely used even now.
However, these alloy materials undergo corrosion or oxidation through reactions with moisture or oxygen in air because of their high activities and chemical unstableness. Such cathode corrosion or oxidation gives rise to considerable growth of non-emitting regions, so called dark spots (D.S.) in the light-emitting layer, and is a cause for characteristic degradation with time of organic electroluminescence devices.
Generally, structural changes due to reactions with moisture or oxygen takes place not only in cathode, but also in organic materials used for the organic thin film layer including the light-emitting layer, the hole transport layer, etc., and thus is likewise a cause for D.S. growth.
As a result of studies on D.S. growth from various viewpoints, the present inventors have found that even such a very small amount of moisture as found in vacuum of about 10xe2x88x924 Torr can promote D.S. growth. To prevent reactions of cathode materials or organic thin film layer materials with moisture or oxygen, thereby completely eliminating D.S. growth and improving durability and reliability of organic electro-luminescence devices, it is necessary to seal the entire organic electroluminescence devices.
For sealing the organic electroluminescence devices, studies have been made so far mainly of the following two procedures. One procedure is to form a protective film on the outer surface of an organic electroluminescence device by vacuum film formation such as vapor deposition, etc., and another procedure is to seal an organic electroluminescence device with a shielding material such as a glass cap, etc.
As to the former procedure for forming a protective film, thereby sealing an organic electroluminescence device, for example, JP-A-6-96858 discloses forming GeO, SiO, AlF3, etc. on the outer surface of the device by ion plating.
Furthermore, JP-A-10-261487 discloses formation of Si3N4, diamond-like carbon film, etc. on the outer surface of the device by ECR plasma CVD.
Still furthermore, JP-A-7-211455 discloses formation of a protective film comprising a water-absorbable material having a water absorption percentage of not less than 1% and a moisture-resistant material having a moisture absorption percentage of not more than 0.1%.
As to the latter procedure for sealing an organic electroluminescence device with a shielding material, the following procedures are available: procedure of providing a glass plate on the outer surface of a back electrode and sealing silicone oil into between the back electrode and the glass plate, as already used in case of inorganic electroluminescence devices; procedure of forming a protective film comprising an insulating inorganic compound, followed by shielding with an electrically insulating glass or electrically insulating hermetic seal fluid (JP-A-5-89959); procedure of sealing a drying agent into a hermetically sealed container (JP-A-6-176867 and JP-A-9-48066), etc.
However, any of these procedures not only fails to meet an increasing demand for a thinner type device, which is characteristic of organic electroluminescence devices, but also fails to meet an expected increasing demand for a film type device. As to sealing procedures which can meet the expected demand for the film type device, procedures of forming a protective film to cover the device have been already proposed. For example, the aforementioned JP-A-10-261487 discloses formation of a diamond-like carbon film or Si3N4, etc. on the outer surface of the device by ECR plasma CVD.
As described above, various procedures have been tried to seal organic electroluminescence devices, and it is desirable from the viewpoint of utilizing an important characteristic of organic electroluminescence devices, i.e. small thickness to conduct sealing only with a thin protective film.
Sputtered SiO2 or Al2O3 has been so far used for a protective film for ordinary electronic devices, but since the organic electroluminescence device is not heat-resistant, film formation procedure accompanying a temperature elevation during the film formation, such as the ordinary sputtering, is not suitable for the organic electroluminescence device.
Vapor deposition, electron beam vapor deposition, ion plating, etc. can be expected as other protective film formation procedures, but the vapor deposition has a problem of porous film formation, and the electron beam vapor deposition and the ion plating have a problem of temperature elevation, and no suitable protective film formation procedure for the organic electroluminescence device has been established yet.
For example, in the protective film formation by ion plating as disclosed in JP-A-6-96858, it is difficult to form a thicker protective film because of temperature elevation during the film formation or internal stress in the film after the film formation, and thus it is impossible to completely suppress D.S. growth.
In the protective film formation by ECR plasma CVD as disclosed in JP-A-10-261487, such a gaseous raw material as SiH4, etc. must be used and the process is so complicated that the cost is inevitably increased. Furthermore, such problems have encountered that in case the gaseous raw material is hardly available, no such a protective film can be formed, either, or the like.
As to the sealing by shielding material tried so far, improvement of shielding materials has been so far made, but no complete suppression of D.S. growth has been attained yet.
As a result of repeated analysis of D.S. growth mechanism, the present inventors have found that there are substances to act as nuclei at the center of D.S., and moisture permeate through the nuclei into the device to promote the D.S. growth. Sizes of the nuclei are in a range of very small submicrons to very large several tens of microns, and to prevent moisture permeation through all the nuclei, a protective film must satisfy the following two requirements: (1) a protective film itself has a low moisture permeability and (2) a thick protective film can be formed. However, all of the aforementioned prior art procedures failed to proposed a protective film satisfying these two requirements (1) and (2).
Furthermore, the present inventors have found that non-emitting regions gradually grow at the edges of the light-emitting surface as similar phenomena to those of D.S. growth. This is because organic materials are exposed at the edges of an organic electroluminescence device and moisture permeates through these exposed edges. Its influence is very large particularly in case that the light-emitting surface is formed in a dot form of high precision. However, a procedure of suppressing such growth of non-emission regions at the edges of the light-emitting surface has not been proposed prior to this invention.
That is, a sealing procedure of completely suppressing D.S. growth in an organic electro-luminescence device, which can satisfy the increasing demands for a thinner type device, a film type device, etc., and also a procedure of suppressing growth of non-emitting regions at the edge of the light-emitting surface have not been established prior to this invention.
An object of the present invention is to solve the foregoing problems and provide an electronic device including an organic electroluminescence device, protected by a dense protective film capable of film formation at a low temperature and preventing permeation of moisture, oxygen etc. into the device from the outside and a process for producing the same.
The present process for producing an electronic device comprises forming a dense protective film at a low temperature by ECR (electron cyclotron resonance) plasma sputtering to prevent permeation of moisture, oxygen, etc. into the device from the outside.
The electronic device according to the present invention is protected by a silicon oxynitride film at least one part of the outer surface of the electronic device, thereby preventing both device damage by the internal stress in the protective film and permeation of moisture, oxygen, etc. into the device from the outside.